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Drought is a major abiotic factor that reduces rice production in rainfed lowland environment. More than 90% of the world'e rice is grown and consumed in Asia. One third of the South and Southeast Asian rice is grown in rainfed lowland environment with shallow rooted indica genotype. In rainfed lowland environment, soils are subjected to compaction and periodic water deficiencies at any stage of the crop growth. Rice root traits such as a high number of penetrated roots, thick penetrated roots, and high root penetration ability through the compacted soil layers play a vital role in the drought resistance mechanism. Breeding for root traits related to drought resistance has been limited due to a lack of suitable techniques for evaluating these traits under field conditions. In this situation, mapping and tagging of drought related root traits using molecular markers is a novel approach for varietal improvement.
The main objective of this study is to identify molecular markers like restriction fragment length polymorphiem (RFLP) associated with total root number, penetrated root number, penetrated root thickness, and root penetration ability of indica rice through wax-petrolatum layers simulated as compacted soil layers. The mapping population of thie study was composed of 166 recombinant inbred lines (RILs) developed from two indica genotypes. This population wae evaluated twice in the greenhouse, in the summer and the fall. Between the two experiments, total and penetrated root numbers were higher in the summer than those of the fall experiment. Favorable environmental conditions, like higher temperature, longer day length, and higher solar radiation contributed to the optimum growth of the plants in the summer season. Data were collected for total root number (TRN), penetrated root number (PRN), and penetrated root thickness (PRT). Root penetration index (RPI) wae derived as the ratio of penetrated root number with total root number. Significant genetic variation was observed for all traits and significant correlation was also found between different traits. The TRN varied from 71-328 in the summer and 45-213 in the fall. The PRN varied from 14-122 in the summer and 6-57 in the fall. The RPI varied from 0.08-0.56 in the summer and 0.06-0.55 in the fall experiment. The PRT varied from 0.40-1.18 mm in the summer and 0.40-1.10 mm in the fall.